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Sensation and Perception ❮ 85Thresholds
Sensory sensitivity can be measured by the absolute threshold,the weakest level of a stim-
ulusthat can be correctly detected at least half the time. For humans, that includes for
sight/vision a candle flame seen at 30 miles on a dark, clear night; for hearing/audition, the
tick of a watch under quiet conditions at 20 feet; for taste/gustation, 1 teaspoon of sugar in
2 gallons of water; for smell/olfaction, 1 drop of perfume diffused in a three-room apartment;
for touch, the wing of a bee falling on your cheek from a distance of 1 centimeter. Have you
noticed that dental or medical procedures feel more painful when you feel tired? It’s not
your imagination! According to signal detection theory,there is no actual absolute thresh-
old because the threshold changes with a variety of factors, including fatigue, attention,
expectations, motivation, and emotional distress. It also varies from one person to another.
Subliminal stimulationis the receipt of messages that are below one’s absolute threshold for
conscious awareness. Subliminal messages can have a momentary, subtle effect on thinking.
Such stimuli can evoke a feeling, though not a conscious awareness of the stimulus. When
you are just barely aware of a change in stimulus, such as an increase in volume of a CD or
brightness on your computer screen, the difference threshold—the minimum difference
between any two stimuli that a person can detect 50% of the time—has been reached. In
order to survive, organisms must have difference thresholds low enough to detect minute
changes in important stimuli. You experience the difference threshold as the just notice-
able difference (jnd).If you add one BB to a container with 10 BBs in it, you’re more likely
to notice a difference than if you add one BB to a container holding 100 BBs. According
toWeber’s law,difference thresholds increase in proportion to the size of the stimulus.
When stimulation is unchanging, you become less sensitive to the stimulus. This sensory
adaptationpermits you to focus your attention on informative changes in your environ-
ment without being distracted by irrelevant data such as odors or background noises.Transmission of Sensory Information
Sensory information of stimuli comes from millions of sensory receptors in your eyes, ears,
nose, tongue, skin, muscles, joints, and tendons. Different receptors detect different types
of physical energy, such as light waves, mechanical energy, chemical energy, and heat energy.
Receptors transduce energy from one form into another. In sensation, transductionrefers to
the transformation of stimulus energy to the electrochemical energy of neural impulses. Except
for impulses for olfaction/smell transmitted directly to the olfactory bulbs on the underside of
the cortex, impulses from sense organs are transmitted to the thalamus before the cortex. The
cerebral cortex puts all the sensory information together and acts on it. Different areas of the
cortex translate neural impulses into different psychological experiences, such as odor or touch.
Visual information is first processed in the occipital lobes in the back of the cortex, hearing
in the temporal lobes, body senses in the parietal lobes, taste at the junction of temporal and
parietal lobes, and smell in the lower portion of the frontal lobes. These primary sensory cen-
ters then project the results of their activity to other areas in the cortex, the association areas,
where more abstract information processing takes place and where you connect new informa-
tion with old information stored in your memory. Perceptionis the process of selecting, organ-
izing, and interpreting sensations, enabling you to recognize meaningful objects and events.Vision
While psychologists study all sensory processes, a major focus is visual perception because
most of us depend so much on sight. Initial visual sensation and perception take place in